Help needed finding oscillator values

You have to play tricks to get a 50% duty cycle. You can parallel the discharge resistor with a diode so that the charge current goes through the diode and the discharge is alone; this gives you about as good a 50% duty cycle as you'll ever get.

You can vary the frequency with a pot, but it'll vary the duty cycle as well. You can fix this with a dual pot, but that limits your capabilities.

Consider using the 555 to drive a divide-by-two counter made from a J-K flip flop. Run the 555 at 60-120Hz, with a relatively small discharge resistance value to give the frequency set pot more authority. Then use the J-K flip flop to give you a perfect 50% duty cycle.

You could even use a 4020, 4040, or 4060 counter and _really_ divide the

555 output down; this would let you run the 555 in the high-audio range where it's happiest, and it would let you use a reasonably small capacitor instead of the monster you'll need at 30Hz.

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Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

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